1. Technical Field
The present disclosure relates to an antenna device using parasitic elements and a radio communication device using the antenna device.
2. Description of the Related Art
A radio communication terminal represented by, for example, a cell phone terminal includes an antenna device which is used for radio communication. In recent years, it is practiced to equip a feed antenna to which the power is fed with a parasitic element to be capacitive-coupled to an antenna element of the feed antenna in order to improve the characteristic of the antenna device.
In addition, a radio communication terminal which includes a plurality of antennas in order to cope with a plurality of communication systems is also proposed. FIG. 1 is a diagram illustrating an example in which in a radio communication terminal including first and second antennas, where a parasitic element is prepared for one (in the example illustrated in FIG. 1, the first antenna) of the antennas. A radio frequency circuit (an RF circuit) is connected with each of the antennas via a corresponding matching circuit.
FIG. 2 is a diagram illustrating an example in which in a radio communication terminal including first and second antennas as in the case in the example illustrated in FIG. 1, parasitic elements are prepared for both of the antennas.
In addition, a so-called multiband antenna in which a single antenna includes a plurality of antenna elements so as to cope with a plurality of frequency bands is also proposed. FIG. 3 is a diagram illustrating an example in which a parasitic element is prepared for one (in the example illustrated in FIG. 3, a first antenna) of antenna elements of a multiband antenna of the type as described above. A single radio frequency circuit is connected with the first and second antenna elements via a single matching circuit.
In order to use a parasitic element in any of the above mentioned antenna devices, it may be unavoidable to prepare an additional parasitic element independently of the antenna elements included in the feed antenna. In addition, it may be unavoidable to prepare an area in which the parasitic element is arranged and a component such as a spring or the like used to connect the parasitic element with the ground.
Japanese Laid-open Patent Publication No. 2005-260762 discloses a communication apparatus that includes first and second antennas respectively coping with first and second working frequency bands. In the above mentioned communication apparatus, two switches which are connected with the both antennas are changed over so as to operate one of these two antennas as a feed antenna and to operate another antenna as a parasitic antenna. That is, in the case that one of the antennas is connected with a radio frequency circuit so as to be used as a feed antenna, another antenna is connected with a ground potential so as to be used as a parasitic antenna.
Japanese Laid-open Patent Publication No. 2007-104637 discloses a radio communication terminal that includes a main antenna and a sub antenna used for diversity reception. In the above mentioned radio communication terminal, switches which are respectively connected with the main and sub antennas are changed over so as to function the sub antenna as an antenna used for diversity reception or as a parasitic element for the main antenna.
Japanese Laid-open Patent Publication No. 2004-274445 discloses a radio device that includes first and second antenna elements coping with a plurality of communication systems. In the above mentioned radio device, phasers that are respectively connected with the first and second antenna elements are provided and controlled using a control unit to adjust the impedance on the side of a circuit viewed from a feeding point of the antenna so as to operate, in feeding one antenna element, another antenna element as a parasitic element.